Fundamentals Of Plumbing:

Re-Routing A Natural Gas
Or Propane Pipe

In This Article:

The gas supply is shut off, then a branch pipe is dismantled and re-assembled in a new location.

Related Articles:
Skill Level: 3 (Moderate) Time Taken: 1 Hour

By , Editor

Intimidated By Natural Gas:

About 10 years ago, when I was working as a design engineer, I bought my first house. I mentioned to a co-worker, a senior mechanical engineer, that I needed to get a plumber to install the flexible gas connector for the stove, because it was something that a homeowner could not, or should not, do himself.  The engineer enlightened me about natural gas.

He explained that the pressure was very low, less than 1/2 of a pound per square inch. But more importantly, he explained the safety of natural gas. All gaseous fuels (and vaporized liquids) have a certain range of air-to-fuel ratios that will allow for combustion. Natural gas, he explained, requires a very precise air-to-fuel ratio.  This fact makes it very unlikely that a minor gas leak could result in an explosion. (But it is possible.) The smell of gas, he told me, would be unbearably strong before the atmosphere became explosive.

His confidence and knowledge convinced me to try connecting the stove myself. As he suggested, I bought some pipe thread compound for the fittings and, when finished, I tested the connections for leaks by dripping some soapy water on them.  I couldn't believe how simple it was.  Later, I began to notice the wide availability of gas pipe, fittings, and flexible connectors. I saw these parts in every hardware store and home center I shopped at. A relative who worked in industrial maintenance confirmed how common it was for average people to do their own gas piping. From that point onward gas plumbing took on a new perspective.


Nevertheless, natural gas can be dangerous, and propane can be very dangerous. They demand respect. Using pipe, fittings, or thread compound that are not rated for natural gas would be foolish and possibly life-threatening.  And all connections must tightened properly and be tested for leaks.

Propane is more dangerous than natural gas for at least two reasons. First, I'm told that propane has a wider range of air-to-fuel ratios that are combustible, so ignition could occur with a smaller leak than would be possible with natural gas.

More importantly, propane is heavier than air, while natural gas (which is mostly methane) is lighter than air. Leaking natural gas will rise by itself, and infiltrate the house where it can be detected by the odor. But if propane leaks it can "puddle" in low spots, and if the puddle reaches a pilot light or a spark from a motor... Kablooey! It has happened.  There are combustible gas detectors available, (like a smoke detector for gas or propane) but I have yet to locate a place to buy them.


The Problem:

This gas pipe is for a Jenn-Air indoor grill. However, the kitchen plans have changed and the pipe is in the wrong position. It needs to be about 10 inches to the right. 
First I laid out the new pieces of pipe. 

My plan was to remove the pipe that protruded through the floor and replace it with a shorter piece, and then install an elbow to bring the pipe under the proper cabinet.

I turned off the gas at the gas meter. This required a large adjustable wrench.
This valve is a ball valve. It only requires 1/4 of a turn to close. It is different from most common valves because it can be rotated forever, it has no internal stop device. It is closed when the bar is perpendicular to the pipe. (Also, two holes line up so the gas company can lock the valve closed.)

Begin Disassembly:

With the gas turned off I could open up the system safely.

I removed the cap with a pipe wrench.
And then I opened the valve slowly, to release the gas pressure. A rushing or whistling sound was heard for a split-second, and then nothing. That told me that the shutoff valve outside was holding properly.


I loosened the vertical pipe that passed through the floor. This piece was 18" long and will be replaced with a 14" section.
In The Basement:  The other end of the pipe.


I removed the pipe.
I replaced it with a shorter section. This pipe is a combination of a 12" piece and a 2" piece. A short piece of steel pipe with threads on both ends is called a nipple. Don't ask me why.

A real plumber would have cut a long piece of pipe to the exact length needed and then make threads on the cut end with a pipe threading tool. But I didn't have that. The fewer joints there are in the pipe system, the fewer places there will be for leaks to develop.

I tightened the new pipe. Note that after this picture was taken, I used a second pipe wrench to hold the elbow. This prevents the pipe assembly from being harmed by the strong twisting forces required for tightening the connections.
Then I wiped the excess thread compound from the new connection. (See below for more info.)


Now the pipe only sticks up a few inches. It is low enough to fit in the toe-kick space below the cabinets.


Connection Basics:

Brushing teflon pipe thread compound on ends of iron pipe. I applied pipe thread compound to the male threads.
The thread compound I used, about $5 at Home Depot.


The thread compound (also called "pipe dope") is applied evenly and liberally. Bare spots are not acceptable.
The fitting is threaded on by hand.


Tightening iron pipe fittings with pipe wrenches. Then a pipe wrench is used to tighten the fitting. A second wrench was later used on the pipe below the fitting, to fully tighten the connection.

How Tight Is Tight Enough?

The degree of tightness is something that takes experience to determine, but I have learned a few general guidelines over the years:

  • If a pipe gets crushed while tightening, the joint is too tight.
  • The best way to properly tighten a joint, in my opinion, is to use two pipe wrenches placed in a tight "V" arrangement, so the handles are close together and a minimum of sideways bending force is applied to the piping.
  • There is a rule of thumb for tightening iron pipe: First, hand-tighten the fitting, and then use wrenches to further tighten the connection at least one turn, and at most two turns.
  • Iron pipe uses a thread system called NPT, which means National Pipe Taper. The thread is cut on a slight taper, so the diameter at the tip of the pipe is smaller than the diameter farther in. Look closely at a pipe thread and this should be obvious. The more a pipe is tightened, the greater the interference fit, and if taken too far the pipe can deform and leak.

The Trick With Threaded Piping:

Getting the elbow fitting to point in the right direction can be tricky. I would never back off a threaded connection to adjust the fitting's direction, because this may cause the thread compound to not cover the threads properly. This means that I have to keep tightening the fitting until it points in the right direction AND it has been tightened adequately. In my early days there were many times when I wondered if the pipe would break off before it finally reached the right position. Then I smartened up.

The solution to this problem is to let the next joint upstream "absorb" some of the tightening. In the above case, I could have had a helper go downstairs and hold a pipe wrench on the next elbow and then two (or more) joints could rotate as I tightened the upstairs elbow. If the upstream joints have only been tightened one turn, then each joint can be rotated another full turn. This allows enough extra rotation of the elbow to let it point in the direction needed.

So... don't over-tighten any piece of pipe. One turn past hand-tight, maybe a little more. Only fittings turn corners (like elbows and tees) so these are the components that may need to be turned more than one full turn past hand tight.


Carrying On:

I installed the next piece of pipe.
And tightened it with two wrenches.


I installed another elbow and a vertical piece.
Note how the thread compound oozes out. This gets wiped off.


I removed the old valve from the 18" pipe, using a 10" adjustable wrench and a pipe wrench.
I installed the valve on the vertical pipe. Note how the pipe wrench is on the elbow below the pipe. This lets two joints absorb the tightening requirement, and gives me more flexibility in locating the position of the blue handle. Also note the "V" arrangement of the wrenches.

At this point, I was done with the piping changes. I went outside and turned on the gas.

But there is air in the line I just worked on, so I briefly opened the gas valve to purge the air and gas from the line. The smell of gas is rather strong.
Then I replaced the cap on the top. Later, an adapter will be installed here when the final flexible gas connection is made to the cooking unit.

The stop valve: There are brass valves widely available that have the letters "WOG" molded into the body. This stands for Water Oil Gas, and can be used on gas lines.


Why Purge The Line?

Natural gas by itself is not explosive or even combustible. It must have oxygen to burn. Any air in the pipe line could allow an explosion if... and it's a big if... a malfunction in another appliance allowed the flame to flash back upstream. This scenario is far fetched, but it only took a few seconds to purge this line. Air does not belong in gas lines.

Obviously, no smoking or open flames should be allowed until the smell of gas has dissipated. Turn off all motorized devices, as many motors create sparks that could ignite gas.

Open a couple of windows or doors while purging a gas line.

Leak Check:

I put a few drops of dishwashing detergent in a cup of water, and dribbled some soapy water on each new connection.

This is a very reliable test. I have seen gas company employees use it. Even a tiny leak will show up as a glob of foam that slowly grows. It takes perhaps a minute or two for a leak to show up.

I wiped off the soapy water when done, to avoid corrosion.

Notes And Warnings:

  • Read our disclaimer.
  • Only use pipe, connectors and thread compound approved for gas piping.
  • Regular Teflon (or TFE) tape is forbidden on gas pipe connections, because small pieces of tape can get shredded during assembly, break off, and flow downstream to block a gas valve. There is a special Teflon tape available for gas piping, but I prefer the liquid compound.
  • Any homeowner who seeks the lowest risk method of connecting a gas appliance or re-routing a pipe should seek the help of a licensed professional plumber or heating contractor.



Tools Used:

  • Pipe Wrenches (2)
  • Adjustable Wrench, 10"

Materials Used:

  • Black Pipe, ½"
  • Black Pipe Fittings
  • Shut-Off Valve
  • Pipe Thread Compound
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Copyright © 2000, 2005

Written January 2, 2000
Revised January 12, 2005